linux-kernel - Re: [PATCH v6] psi: fix race between psi_trigger

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Message-ID: <YMHAEQMVhGhveGeE@hirez.programming.kicks-ass.net>
Date:   Thu, 10 Jun 2021 09:32:33 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     Huangzhaoyang <huangzhaoyang@...il.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Zhaoyang Huang <zhaoyang.huang@...soc.com>,
        Ziwei Dai <ziwei.dai@...soc.com>, Ke Wang <ke.wang@...soc.com>,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>
Subject: Re: [PATCH v6] psi: fix race between psi_trigger_create/destroy

On Thu, Jun 10, 2021 at 09:31:33AM +0200, Peter Zijlstra wrote:
> On Fri, May 21, 2021 at 10:18:53AM -0400, Johannes Weiner wrote:
> > On Fri, May 21, 2021 at 10:05:54AM +0800, Huangzhaoyang wrote:
> > > From: Zhaoyang Huang <zhaoyang.huang@...soc.com>
> > > 
> > > Race detected between psi_trigger_destroy/create as shown below, which
> > > cause panic by accessing invalid psi_system->poll_wait->wait_queue_entry
> > > and psi_system->poll_timer->entry->next. Under this modification, the
> > > race window is removed by initialising poll_wait and poll_timer in
> > > group_init which are executed only once at beginning.
> > > 
> > > psi_trigger_destroy                      psi_trigger_create
> > > mutex_lock(trigger_lock);
> > > rcu_assign_pointer(poll_task, NULL);
> > > mutex_unlock(trigger_lock);
> > > 					mutex_lock(trigger_lock);
> > > 					if (!rcu_access_pointer(group->poll_task)) {
> > > 
> > > 						timer_setup(poll_timer, poll_timer_fn, 0);
> > > 
> > > 						rcu_assign_pointer(poll_task, task);
> > > 					}
> > > 					mutex_unlock(trigger_lock);
> > > 
> > > synchronize_rcu();
> > > del_timer_sync(poll_timer); <-- poll_timer has been reinitialized by
> > > psi_trigger_create
> > > 
> > > So, trigger_lock/RCU correctly protects destruction of group->poll_task but
> > > misses this race affecting poll_timer and poll_wait.
> > > 
> > > Fixes: 461daba06bdc ("psi: eliminate kthread_worker from psi trigger
> > > scheduling mechanism")
> > > 
> > > Signed-off-by: ziwei.dai <ziwei.dai@...soc.com>
> > > Signed-off-by: ke.wang <ke.wang@...soc.com>
> > > Signed-off-by: Zhaoyang Huang <zhaoyang.huang@...soc.com>

Argh noticed the SoB chain is invalid. Please fix.

> > 
> > Acked-by: Johannes Weiner <hannes@...xchg.org>
> 
> Thanks!